Method of producing heat-treated cast iron alloys



April 2, 1940 w. J. SPARLING mama 0F rnonucme HEAT-TREATED CAST momALLOYS Filed March 22, 1959 FERRITE FERRITE GRAIN o v 0 0 fl wi b 0 M wm w O M6 00 60 0 W A0 0 m o o 9 D a my GRAPHITE BOUNDARIES FERRITE GRAINBOUNDARIES CEMENTITE WHICH HAS NOT MIG-RATED MATRIX ERLO CKING FERRITEGRAIN BOUNDARY AREAS rm mm: ms

AND FORMED AN INT UNIFORMLY DISTRIBUTED THROUGHOUT THE SPHEROIDIZEDCEM'ENIITE SUBSTANTIALLY FERR ITE SPHEROIDIZED CEMENT MIGRATED T0NETWORK IN TEE k LQEQQEER ZOO . INVENTOR.

TIME HOURS Patented Apr. 2, 1940 UNITED STATES METHOD OF PRODUCINGHEAT-TREATED C ST IRON ALLOYS ,William J. Sparling,

Milwaukee, Wis., assignor to Chain Belt Company, corporation ofWisconsin Milwaukee, Wis., a;

Application March 22, 1939, Serial No. 263,445 8 Claims. (oi. mil-21.8)

This invention relates to heat-treated white cast iron alloys, and moreparticularly to improvements in the process of producing heattreatedwhite cast iron alloy castings of the type disclosed in my Patent No.2,119,833, granted June 7, 1938.

It is among the objects of the invention to provide a process forproducing heat-treated white cast iron alloy products of the typedisclosed in my aforesaid patent which affords substantial operatingadvantages and economies as compared with the process disclosed andclaimed in that patent while producing products of similar structure andutility.

The invention will be described-with reference to the accompanyingdrawing in which Fig. l is a diagrammatic representation of themicrostructure of a casting at one stage of the process; Fig. 2 adiagrammatic representation similar to Fig. 1 showing the microstructureof the finished product; and Fig. 3 a graph representative of thetwo-stage heat treatment provided by the invention.

The term "white cast iron is applied in the industry to a cast ferrousmetal containing appreciable amounts of carbon and silicon which are soproportioned to one another that in a casting made from the alloy thecarbon does not separate as flake graphite but is largely or wholly incombined form, i. e., as cementite, and the casting exhibits asilver-white appearance when fractured, which characteristicsdistinguish it from gray cast iron which contains substantial amounts ofcarbon separated from the alloy during solidification and cooling. Whiteiron castings are usually subjected to a malleableizing heat treatmentto produce phase changes I that develop desirable mechanical propertiesin the heat-treated products.

Although in white cast iron the contents of carbon and silicon, as wellas those of elements incidentally present such, for instance, asmanganese, sulfur and phosphorus, vary somewhat,

white cast iron as commonly made for mal- 0 leableizing may be said tohave an average composition approximately within the following limits:

Per cent Carbon 1.90' to 3.00 Manganese 0.20 to 0.40 Phosphorus 0.08 to0.16. Silicon 1.30 to 0.60- Sulfur 0.05 to 0.12

I The invention of my aforesaid Patent No. 2,119,833 is predicated uponthe application of a novel heat treatment to castings of white cast ironalloys containing manganese and copper in excess of the amounts used inthe art prior to that invention, which treatment and compositioncooperate to provide articles of novel structure' and improvedproperties. In accordance with that invention castings are made from analloy cast iron adapted to produce white iron castings and containing atleast about 0.6 per cent of manganese together with at least about 0.5per cent of copper. In the preferred practice of that invention theWhite cast iron is preferably of the composition range stated above,containing carbon and silicon in amounts such as to produce a white ironcasting, and it contains from about 0.6 to 1.1 per cent of manganese,and about 0.5 to 1.5 percent of copper.

Prior to the invention of my patent the white cast iron industry hadpreferred low manganese irons, containing less than 0.6 per cent ofmanganese, because of the tendency of manganese to interfere withmalleableizing. Copper was also avoided because of its so-calledsoftening effect, or where it was used chromium was added also tocounteract the undesired effect of copper. In the practice of my'patented invention the copper and manganese so cooperate under theinfluence of the heat treatment of the invention that" their undesirableeffects are not observed, and the use of chromium is unnecessary.

Such white cast iron alloy castings are heat treated, in accordance withmy patented invention, by heating them to a temperature above thecritical point (A1), e. g., above 1500 F. and most suitably at about1600 to 1800 F., for a period of time sufiicient to decompose all of themassive cementite with production of an austenitic matrix containingfinely divided graphitic carbon. After decomposition of the cementitehas been completed the casting is cooled to a temperature well below thecritical point (A1), as by quenching it in air, water, oil, or othersuitable medium to a temperature of, for example, about1200 F. Thecasting is then reheated to a temperature approaching but somewhat belowthe critical point, say 1250 to 1300 F., where it is held for asuflicient length of time to cause spheroidizing of the pearliticcementite. ,The structure of the spheroid-ized product is showndiagrammatically in Fig. 1. It comprises a ferrite matrix through whichspheroidized cementite is substantially uniformly distributed, togetherwith some proportion of finely divided graphitic carbon produced in thedecomposition of the massive cementite.

After thegcasting has been thus spheroidized the temperature is raisedto within the critical range, say between 1325 and 1360 F. and held atthat temperature for a period of time such that in consequence of theuse of copper and manganese, the spheroidized cementite undergoes arearrangement in which a substantial proportion of it appears in theferrite grain boundaries in the form of an interlocked structure. Thearticle is then cooled to preserve that structure which is a novelfeature of the patented invention and the cause of the remarkableproperties of the castings. The structure and properties are obtained,as far as I am aware, only by the use of copper and manganese in theamounts stated conjointly with final heating of the spheroidized articlein the critical range. The structure of the product is showndiagrammtically in Fig. 2.

Articles produced in accordance with my patented invention possessvarious properties which render them substantially superior to ordinarymalleableized white iron castings of normal composition. For example,malleableized white iron castings of the composition stated hereinabovemay be expected to exhibit mechanical properties approximately withinthe following limits:

Tensile strength, psi "45,000 to 60,000 Elastic limit, psi 32,000 to38,000 Elongation in 2" per cent 25 to 10 Hardness, Brinell .115 to 140Impact value, ft. lbs 15 to '7.

In contrast, a casting made in accordance with my patented inventionfrom a cast iron alloy containing 2.29 per cent of carbon,,1.00 per centof copper, 0.85 per cent of manganese, and 0.83 per cent of siliconwhich had been heated to 1700 F. for thirty hours, quenched below thecritical temperature, reheated to 1270 F. for thirty hours, and finallyheated to 1340 F. for five hours, followed by quenching, possessed thefollowing mechanical properties: Tensile strength, s1 "90,700 Elasticlimit, psi ...56,300 Elongation in 2" per cent..- 13.5

' In general, products made in accordance with the process of my patentwill possess mechanical properties about as follows:

Tensile strength, psi --67,500 to 110,000 Elastic limit, psi 47,500 to05,000 Elongation in 2". per cent 18 to 12 Hardness, Brinell 170 to 220Impact value, ft. lbs -30 to 15 Although the products provided by thepatented invention possess substantiallyimproved strength, one of themajor features of that in-' notching the surface of a malleable casting,or

of a casting made from my special alloy, more appreciably decreases theimpact value than is the case with steel test bars.' However, thespecific figures given herein for impact values are representative ofthe impact strengths and are comparable to standard Charpy valuesalthough not to the same scale because of the differences in size andsection of the specimens. The figures given for impact are, however,comparable between themselves.

In addition, the products of my patented invention furthermore possessother improved properties, such as improved resistance to corrosion,which are set forth in detail in the said patent to which reference maybe made for further details and further data concerning the chemical,mechanical and physical properties.

Extensive commercial practice of my patented invention has demonstratedadmirably its ability to produce heat-treated white cast iron alloyproducts of the type described in the patent and summarized hereinabove,and the products are recognized as being novel and possessing propertiessuperior to those attainable theretofore in malleableized products.

The heat treatment disclosed in my aforesaid patent differs from that ofordinary malleable iron practice in that accurate control of temperatureis necessary inasmuch as the process involves heating close to but belowthe critical range followed by raising the temperature relativelyslightly to within the critical range. The factor of temperature controlis therefore more critical in the practice of the patented process thanis the case with ordinary malleableizing treatments. Additionally, it isnecessary to conduct the process in a non-oxidizing atmosphere tominimize surface decarburization. In consequence of such factors theprocess of my patent No. 2,119,833 requires performance in electric orsimilar accurately temperature-controlled furnaces which areconsiderably more expensive to build, operate and maintain than are thefurnaces commonly used in ordinary malleable iron practice. Also, toobtain the best results through the practice of the patented inventionit is desirable to heat treat at one time castings of relatively thesame section, rather than to heat treat together castings of widelyvarying section. Experience has shown that the best results are obtainedby segregating for heat treatmentcastings up to inch section, castingsfrom about to inch section, and, castings having a sectiongreater thaninch. Consequently, although my patented invention performssatisfactorily to provide desirable products as described in the patent,the heat-treating costs are high not only because of the overhead andop- J crating costs inherent in the use of electric furnaces, orsimilarly accurately temperature-controlled furnaces, but also inconsequence of the necessity for either holding castings of a givensection group until enough are accumulated for an entire heat, or ofoperating the furnace with less than a full load.

I have discovered, and it is upon this that my invention is predicated,that all of the benefits of the patented invention may be obtained whilesubstantially reducing production costs by subjecting the castings to amodified heat treatment in two stages. The first stage consists of anordinary malleableizing heat treatment conducted in the manner and withequipment standard in the malleable iron art. In the second stage themalleableized articles are heated to a temperature to cause carbon to beredissolved, or recombined, after which they are cooled to a temperaturebelow the critical range for a time to effect spheroidizingyand finallythe temperature is increased to within the critical range to causespheroidized cementite to become rearranged and interlocked along thegrain boundaries. The castings are then cooled. Hence in the secondstage of the heat treatment of the present invention the castings arespheroidized substantially uniformly and then caused to develop astructure in which a substantial proportion of the spheroidizedcementite becomes rearranged and interlocked along the grain boundaries.

The products produced by the two-stage heat treatment of the presentinvention thus possess a composition and microstructure which providemechanical, physical and chemical properties the same as those ofproducts produced in accordance with the patented invention. However,material and valuable economies result from the practice of the presentinvention because a large portion of the expensive heat treatment cycleof the patented invention is replaced by the much less cost- 1y ordinarymalleableizing treatment, with major benefits that will be detailed morefully hereinafter.

As indicated hereinabove, in the practice of the present inventioncastings are made from white cast iron alloy as described in myaforesaid patent. the alloy containing at least about 0.6 per cent ofmanganese and at least about 0.5 per cent of copper, most suitably fromabout 0.65 to 1.1 per cent of manganese and from about 0.5 to 1.5 percent of copper. The best results are obtained with castings in which allof the carbon is in combined form, i. e., in which there is no graphiticcarbon, or substantially none, in the casting at the start of themalleableizing treatment.

Such castings are then heat treated in accordance with the presentinvention. The first stage of the heat treatment is conducted in generalaccordance with standard malleableizing practice. That is, the castingsmay be, and preferably are, packed in iron pots with sand, cinders orother packing material in accordance with practice Which is quite wellstandardized. The pots are then heated in a furnace of the type used formaking ordinary malleableized castings such, for example. as periodicfurnaces fired with powdered coal. Such furnaces are much cheaper tobuild and maintain, and the operating expense is much lower, thanelectric furnaces. this malleableizing stage is to cause decompositionof all of the massive cementite followed by more or less completedecomposition of pearlite. although for the purposes of the presentinvention complete decomposition of the pearlite is unnecessary and ismuch less critical than in ordinary malleable iron practice. At the endof this stage the castings have a structure containing ferrite andgraphite, with or without pearlite depending on the rate of coolingafter decomposition of massive cementite.

At any time after the end of the malleableizing cycle the castings areready for the second stage of the heat treatment. In this stage themalleableized castings are placed in a furnace of the type used in thepractice of the patented invention and heated to a temperature such asto cause carbon to be recombined. as by heating them to, for example,1400 to 1700" F., or higher. higher the temperature in this step, themore rapidly will the maximum amount of carbon be recombined. In thisstep carbon is dissolved up to the eutectoid composition in theaustenite, which contains the excess carbon distributed thereth.ough inthe form of graphite. For most purposes heating for a period of abouttwo hours will condition the casting for the subsequent steps of thisstage of the heat treatment.

The function of The After the maximum amount of carbon has beenrecombined the treatment is the same as in the patented process afterdecomposition of the massive cementite. That is, the castings are thenquenched in any suitable medium, such as air, water, or oil, to bringthem to a temperature Well below the critical range, for instance toabout 1200 F. They are then reheated to a temperature somewhat below thecritical range, suitably 1250 to 1300 F., preferablyto approximately1270 F., and held there for a sufiicient time to spheroidize thecementite, which is now in pearlitic form. This results in theproduction of a structure like that shown in Fig. 1, in which thecementite is substantially completely spheroidized and substantiallyuniformly distributed throughout the entire matrix. During this stepthere is no substantial decomposition of the pearlite into ferrite andgraphite, apparently because this is prevented by the action of therelatively high content of manganese.

After spheroidization, the time period of which A will vary with thesection of the articles and the exact composition of the alloy, butwhich usually ranges between ten and thirty-five hours, the temperatureis raised to within the critical range, suitably between 1325 and 1360F., to cause a substantial proportion of the spheroidized cementiteparticles to rearrange into an interlocked structure along the ferritegrain boundaries. This structure, shown in Fig. 2, characterizes thefinal product of both the patented and the present invention.

The article is then cooled at a rate such as to preserve the structure,as by quenching it in air, water, oil, or other medium with suificientrapidity to retain the structure produced through heating within thecritical range.

'A heat treatment typical of the present invention is illustrated by thegraphs of Fig. 3. The first stage represents a standard, orconventional, malleableizing treatment conducted in an ordinary periodicmalleableizing furnace. The castings are heated during, for example,forty hours to a temperature of about 1600 F. which is maintained longenough to cause decomposition of all of the massive cementite, sayduring about fifty hours. The articles are then cooled to roomtemperature. In the embodiment shown the cooling is slow, requiringabout eighty hours to reach room temperature. t permit decomposition ofpearlite.

The actual times and temperatures used in the first stage may vary inpractice from those shown in Fig. 3. Thus, the castings may be heatedabove the critical range more rapidly or more slowly than shown, andthey need be held at temperature only long enough to effect breakdown ofall of the massive cementite. Also, the temperature of l600 F. for thispart of the first stage is not critical, it being possible to effectbreakdown of cementite at lower or higher tempera tures, say l500 to1800 F. The decomposition wilLbe accelerated by increase of temperature,but 1600 F. is chosen because it represents an average value productiveof economically rapid decomposition.

Similarly, although the first stage is represented as being inaccordance with standard malleable practice, it is not essential for thepurpcses of the invention that the articles be fully malleableized. Thatis, it is not essential that the articles to held below 1400 F. orcooled slowly to break up pearlite, nor is it necessary that theyconsist essentially of ferrite and graphite. Provided the massivecementite be completely decomposed it is entirely feasible in thepractice of this invention for the articles to contain a greater orlesser proportion of pearlite at the end of the first stage. Hence thearticles may be cooled more rapidly from above the critical range thanis indicated in the first stage of Fig. 3. Thus, they might be cooled asshown in the broken line of the first stage graph, say by allowing themto cool without refiring the furnace, or by hauling the pots from thefurnace for air cooling, thus reducing the time involved in this stage.

As thus malleableized the articles are ready for the second stage of thetreatment in accordance with the present invention. This stage of theheat treatment may, of course, be applied at any time aftermalleableizing. In the embodiment of the second stage graph of Fig. 3,the articles are heated to a temperature and for a period of time toeffect recombination of carbon. They are then cooled, suitably byquenching, to a temperature well below the critical range, shown in Fig.3 as being 1200 F., followed by reheating to a temperature close to butbelow the critical range, 1275" F. in the embodiment shown, and held atthat temperature to effect spheroidization as described hereinabove.Finally, the articles are heated to within the critical range, say 1340F., followed by cooling at a rate such as to preserve the structureproduced in the last step. As in the case of the first stage and asexplained in my aforesaid patent, the exact times and temperaturesapplied in the second stage may vary from those shown in Fig. 3 which isincluded purely by way of illustration and not by way of limitation.

In the practice of the process of the patented invention a completecycle in the electric furnace may require between ninety and one hundredhours. In the present invention only the second stage of the heattreatment is performed in the electric furnace, and for most sizes ofcasting this may be completed in about forty to sixty hours. Thus theelectric furnace cycle is reduced to about one-half or two-thirds ofthat necessary heretofore. This results in major economies, and this isso even though the malleableizing cycle of the first stage be ofconsiderable duration because experience has shown that themalleableizing stage can be performed much more cheaply than bydecomposing the cementite in an electric furnace cycle.

Other economies result from the fact that in the malleableizing stagecastings of all sizes and sections may be treated at once so that it ispossible always to run the malleableizing furnace at full load. Themalleableized castings may then be segregated into groups of appropriatesections for treatment in the second, or electric furnace, stage. Inthis manner it is possible to accumulate the various section sizes,through operation of the malleableizing furnace at full load on allsections, so that full loads are always available for the electricfurnace cycles.

In this manner also the capacity of the electric furnaces issubstantially doubled because the electric furnaces are freed from thefirst half of the cycle used in the patented process so that they can becontinuously operated for the second stage only, thus effectivelydoubling the production capacity of a given electric furnaceinstallation. This further reduces operating costs because if the entireoperation be conducted in electric furnaces twice the capacity would berequired that is necessary in practicing the present invention, withdoubled first, upkeep and operating costs.

- A major advantage of the present invention is that any warpage which acasting is likely to undergo will be substantially completed in thefirst stage. In consequence of its malleableized condition the castingmay then be straightened easily prior to being subjected to the secondstage. The importance of this feature arises from the fact that it isnot possible satisfactorily to straighten articles which may have warpedduring the heat treatment of the patented invention because their hightensile strength and hardness after heat treatment are comparable incertain respects, especially as to straightening of warped castings, tohigh carbon steel. This ability to straighten the castings in thepresent invention results in greater output through reduction in theproportion of castings which may have to be rejected because of warpageand allied defects.

According to the provisions of the patent statutes, I have explained theprinciple and method of practicing my invention and have illustrated anddescribed what I now consider to represent its best embodiment. However,I desire to have it understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallyillustrated and described.

I claim:

1. That method of treating castings of white cast iron alloy containingin excess of about 0.5 per cent of copper and in excess of about 0.6 percent of manganese, which comprises subjecting said casting to amalleableizing treatment to effect decomposition of massive cementite,then reheating the malleableized castings to temperatures successivelyabove and below the critical range for times to effect, respectively,solution of carbon and formation of spheroidized, cementite, and thenheating within the critical range for a time to cause rearrangement ofthe spheroidized cementite to produce at the grain boundaries aninterlocked structure of spheroidized cementite.

2. That method of treating castings of white cast iron alloy containingin excess of about 0.5 per cent of copper and in excess of about 0.6 percent of manganese, which comprises subjecting said castings to amalleableizing treatment involving heating the castings to a temperatureand for a time to effect decomposition of massive cementite, and coolingthe castings to room temperature; then reheating the malleableizedcastings to a temperature above the critical point for a time to effectsolution ofcarbon, then at a temperature below the critical range toeffect formation of substantially uniformly distributed spheroidizedcementite, and then heating within the critical range for a time toproduce at the grain boundaries an interlocked structure of spheroidizedcementite, and cooling the castings at a rate to preserve saidstructure.

3. A two-stage method of treating castings of white cast iron alloycontaining in excess of about 0.5 per cent of copper and in excess ofabout 0.6 per cent of manganese, which comprises, as a first stage,heating said castings to a temperature between about 1500 and 1800 F.for a time to effect decomposition of substantiallyall of the massivecementite, and cooling the castings to room temperature at a rate suchas to effect decomposition of pearlite with formation of ferrite andgraphite; then, as the second stage, reheating the malleableizedcastings to a temperature above about 1400" F. for a time to effect sotoa malleableizing treatment, then reheating the malleablized castings toa temperature and for a time to effect solution of carbon, cooling thecastings to a temperature below the critical range and heating for atime and at a temperature adjacent but below the critical range toproduce substantially uniformly distributed spheroidized cementite, andthen raising the temperature into the critical range for a time to causespheroidized cementite to form an interlocked structure along the grainboundaries, and cooling at a rate to preserve said grain boundarystructure.

5. That method of heat treating white cast iron alloy castingscontaining about 0.5 to 1.5 per cent of copper and about 0.6 to 1.1 percent of manganese, which comprises subjecting said castings to amalleableizing treatment, to decompose substantially all of the massivecementite, cooling the castings to room temperature, then heating themalleableized castings to a temperature of about 1300 to 1700 F. for atime to effect solution of carbon, cooling the castings to a temperaturebelow the critical range and heating for a time and at a temperature ofabout 1200 to 1300 F. to produce substantially uniformly distributedspheroidized cementite, and then raising the temperature into thecritical range for a time to cause spheroidized cementite to form aninterlocked structure along the grain boundaries, and cooling at a rateto preserve said grain boundary structure.

6. That method of heat treating white cast iron alloy castingscontaining about 0.5 to 1.5 per cent of copper and about 0.6 to 1.1 percent of manganese, which comprises subjecting said castings to amalleableizing treatment to decompose substantially all of the massivecementite, cooling the castings to room temperature, then heating themalleableized castings to a temperature above about 1400 F. for a timeto efiect solution of carbon, then cooling the castings to a temperaturebelow the-critical range and heating them for a time and at atemperature between about 1200 and 1300 F. to produce substantiallyuniformly distributed spheroidized cementite, and then raising thetemperature to between about 1325" to 1360 F. for a time to causespheroidized cementite to form any interlocked structure along the grainboundaries, and cooling at a rate to preserve said grain boundarystructure.

7. That method of heat treating white cast iron alloy castingscontaining about 0.5 to 1.5 per cent of copper and about 0.6 to 1.1 percent of manganese, which comprises heating said castings to atemperature for a time to decompose substanstantially all of the massivecementite, cooling the castings, then heating the castings to atemperature above about 1400 F. and below about 1800 F. for a time toefi'ect solution of carbon, then cooling the castings to a temperaturebelow about 1300 F. and heating for a time and at a temperature of about1250 to 1300 F. to produce substantially uniformly distributedspheroidized cementite, then raising the temperature to between about1325 to 1360 F. for a time to cause spheroidized cementite to form aninterlocked net work structure along the grain boundaries, and coolingat a rate to preserve said grain boundary structure.

8. That method of treating castings of white cast iron alloy containingin excess of about 0.5 per cent of copper and in excess of about 0.6 percent of manganese, which comprises subjecting said casting to amalleableizing treatment involving heating the castings to a temperatureand for a time to effect decomposition of massive cementite, and coolingthe castings; straightening warped castings after such malleableizingtreatment; then reheating the malleableized castings to temperaturesabove and below the critical temperature for times to form substantiallyuniformly distributed spheroidized cementite, and then heating withinthe critical range for a time to produce at the grain boundaries aninterlocked structure of spheroidized cementite.

WILLIAM J. SPARLING.

